What is Minecraft Indev?

Today I was watching a video of iDeactiveMC and found out that there is a old version of Minecraft called Minecraft Indev.

Please explain me exactly what Minecraft Indev is?

Indev, literally meaning "in development", was the pre-beta version of Minecraft between December 2009 and February 2010. It was succeeded by Infdev, and then Alpha.

Basically, Minecraft development moved through a number of phases.

There was:

• The "pre-classic" era in mid 2009. The game was strictly single player creative.
• The multiplayer and survival tests a few months later.
• Indev, starting in early 2010 (version 0.31, though the version number wasn't being bumped to match releases for the later part of this)
• Infdev through mid 2010, which added infinite world generation (there used to be a hard limit on world sizes)
• Alpha, after June 2010, starting with 1.0.0. As I understand it, this was the point when people could actually buy the game.
• Beta 1.0 starting in December 2010, and going until November 2011
• And finally the release series, starting at 1.0.0 and stretching on to today.

There's not a whole lot to explain beyond this and the linked wiki article :)